1. Field of the Invention
This invention relates to monitoring of tires, in particular to circuits and devices in which the transmission of an electrical monitoring signal occurs by electric conduction between the wheel and the vehicle, while transmitting sufficiently valuable information to make an effective monitoring possible.
2. Discussion of the Background
In U.S. patent application Ser. No. 07/111,349, it was proposed to provide the transmission of an electrical signal by electric conduction between a wheel of a vehicle and the vehicle itself, i.e., between the stationary space of the vehicle and the mobile space in rotation. For this purpose, at least one of two electrically conductive paths, necessary to provide the transmission of an electrical signal, passes through a vehicle bearing. However, a bearing constitutes a mechanical part through which electric conduction is not perfect. It introduces into the electric line a spurious resistance whose value cannot be controlled and which fluctuates in a rather wide range as a function particularly of heating and transmitted stresses. This poses no major problem if the electrical signal transmitted is of the "all or nothing" type. Therefore, for example, a simple pressure switch is installed on the tire.
Unfortunately a simple pressure switch does not make it possible to take sufficiently into account the operating condition of a tire. Temperature is an important parameter that must be taken into account. Further, it is desirable to be able to distinguish a slow pressure loss, which undoubtedly requires handling the tire gently, for example by driving at a slow speed, from a rapid pressure loss, which would make immediate stopping necessary.